Cells in advanced organisms don't divide continuously but rather in a planned, coordinated fashion. Young organisms grow in a controlled way and the cells of mature organisms don't divide as often. To achieve this coordination, cells use both external and internal factors to decide when to divide.
In the cell cycle, cells spend most of their time in the interphase stage where they carry out specialized functions and grow. When internal or external factors that influence cell division tell them to divide, they go through several stages to prepare. In each stage, they can stop the division process depending on the factors that are present.
Internal factors that affect cell division are especially important because they help make sure cells divide only when new cells are needed by the organism. Such factors include chemicals present in the cell itself and chemical triggers resulting from the signals of other cells. These chemicals influence how the cells and the organism grow and behave.
The Cell Cycle Governs Cell Division
The cell cycle is made up of the part in which the cell actually divides and the interphase, or part in which the cell is not ready for division or is making preparations for it.
The four major stages of the cell cycle are as follows:
- Gap 1. The cell has successfully divided and the two new daughter cells are ready to take up their roles in the organism. Most cells spend almost all their time in this stage.
- Synthesis. The cell has decided to divide and replicates its DNA so it will have the required two copies of each chromosome.
- Gap 2. The cell is prepared to divide but has to check to make sure everything is ready. DNA integrity, the presence of enough cell material and the verification of signals from other cells is carried out.
- Mitosis. The chromosomes and the nucleus divide. The organelles are shared out and the cell grows a new dividing membrane. Two identical daughter cells are created.
The points at which external and internal factors can influence the cell cycle and the cell division process are located throughout the gaps and mitosis. These checkpoints allow chemical signals and other factors to stop further progress. These are the factors that control the cell cycle and cell division.
Environment and Disease Can Trigger Internal Factors
The two main characteristics that cells verify during checkpoints are whether the cell has enough material on hand to divide into two functional daughter cells and whether the cell DNA is undamaged. While both these factors are internal to the cell, they can be influenced by outside factors.
Typical external factors that influence cell division are the following:
- Availability of raw materials can affect cell division. If not enough nutrients are available, the cell can't grow enough and will not divide.
- Radiation can change DNA molecules. If the DNA has incorrect sequences, the cell will either wait and repair the DNA, stop dividing or enter cell apoptosis or cell death.
- Toxins can damage cell DNA. Such damage will be detected at the checkpoints and the cell will stop dividing.
- Viruses replicate by hijacking a cell's metabolism to make copies of the virus, but viruses can also affect cell DNA. If such anomalies are detected at a checkpoint, the cell will not divide.
- Drugs can affect cell division. For example, cancer drugs influence cell division by blocking internal factors or actions essential for cell division to proceed.
Such environmental influences affect internal factors and through them influence cell division. The cell may stop dividing while it repairs or fixes problems. In some cases cells can then resume the cell cycle and cell division process while in other cases the cell will not divide.
Internal and External Regulators Directly Influence Cell Division
The organism has internal and external regulators that coordinate cell division within specific organs or tissues. For example, some skin cells divide continuously to replace worn and dead skin cells sloughed off from the surface of the skin. Internal and external regulators tell skin cells within a lower skin level to divide if more skin cells are needed.
Such regulators include the following:
- Growth hormone. Controls the growth of cells in young organisms but then cuts back growth when the organism reaches mature size.
- Density-dependent cell signaling. If there are cells sending signals from all sides, a cell may stop dividing. If there are no signals on one or more sides, the cell could keep dividing.
- G1 checkpoint. The cell checks to make sure it is ready to start the division process. If not, the cell may put off dividing, grow some more or stop dividing altogether.
- G2 checkpoint. DNA replication is complete and the cell is ready to split. The DNA molecules are checked for completeness and accuracy. If there is a problem, the cell tries to fix it or it may stop the division process.
- M checkpoint. Mitosis has started and this is the last chance to delay or stop cell division. The cell checks that the correct DNA molecules have separated and are ready to form two cells.
Factors internal to the organism play a key role in determining whether a cell starts dividing and whether it divides successfully. Other cells send signals and the cells that are ready to divide react. The checkpoints themselves are controlled by chemicals internal to each cell.
Kinases and Cyclins are Internal Factors that Regulate Division
When cells reach a checkpoint within the cell cycle, whether they continue division or abort the process is regulated by cyclin-dependent protein kinases. Kinases are present in the cell while the concentration of cyclins rises and falls with the cell cycle. The cyclins activate the kinases.
Kinases have a signal-integration function for internal cell signals such as the presence of damaged DNA or the presence of specific nutrients. If the right signals are present, the kinases are activated by the cyclins and the cell passes the checkpoint. If a blocking signal is present or a required signal is absent, some of the kinases may not be activated and the cell stops dividing.
When Cell Division Goes Wrong
Cell division is tightly controlled because, if something goes wrong, cells can stop dividing when new cells are needed or they can continue to divide uncontrollably. In that case, the organism can develop tumors or diseases such as cancer.
The internal factors that influence cell division such as cell signals and cyclin-dependent kinases are themselves regulated by the organism's genetic code. Genes allow cells to produce the required proteins and hormones used to regulate cell division.
If a gene mutates or suffers damage, substances that would normally stop cell division can no longer be produced and cells may keep dividing when the are not needed. Different kinds of cancer occur when such unwanted cell masses become malignant and send out tumor cells to other parts of the body.
The internal regulators of cell division keep tissue growth in check and and direct cells to divide as needed. They are a key part of a healthy organism, directing growth to maturity and then only to replace lost or damaged cells.